The Centers for Disease Control estimates 1.7 million people suffer traumatic brain injury (TBI) in the United States each year and 5.3 million people are living with TBI-related disability. In survivors, chronic neuroinflammation has been linked to progressive neurologic deficits and neurodegenerative disorders. The cause of persistent neuroinflammation, in some cases years after the primary injury, remains unknown. We propose that chronic intestinal dysbiosis, specifically the depletion of ?healthy? commensal bacteria capable of fermenting dietary fiber to produce the short chain fatty acid (SCFA) acetate, leads to a stable maladaptive state in the brain and neurodegeneration after TBI. Acetate, a glial specific substrate, is the most abundant SCFA in the peripheral circulation, crosses the blood-brain barrier, and plays a critical role in development and function of the immune system. Restoration of depleted bacteria or their metabolites has the potential to reverse dysbiosis-associated phenotypes. Through a unique collaboration between internationally recognized centers for TBI and microbiome research, The Safar Center for Resuscitation Research, the Center for Microbiome and Medicine, and the Brain Care Institute at the University of Pittsburgh, we have generated exciting preliminary data which demonstrates (1) bacterial populations in the gut that produce acetate by fermentation of dietary fiber are depleted after severe TBI in children and controlled cortical impact (CCI) TBI in mice, (2) acetate production is significantly reduced after CCI, and (3) surprisingly, commensal bacterial populations and acetate production do not recover by 28 days after injury, the latest timepoint assessed in our preliminary studies. Thus, we propose a translational study to discover if persistent inflammation-mediated neurodegeneration after TBI is fueled by depletion of commensal bacteria and deficient microbial production of acetate. In Aim 1, we will determine whether acetate repletion using a chow enriched with acetylated fiber or replenishing acetate-producing bacteria in the gut prevents late neurodegeneration after TBI. Cognitive function will be assessed. Lesion volume will be assessed by serial T2-weighted MRI. Surviving neurons will be quantified using unbiased stereology. In Aim 2, we will determine the impact of acetate repletion on chronic microglial activation after TBI by immunohistochemistry and use RNA-Seq to determine microglia phenotype. We hypothesize that mice randomized to receive acetylated fiber or acetate-producing bacteria will have reduced microglial activation in the peri-contusional cortex and assume an anti-inflammatory and tissue-supportive phenotype. In Aim 3, we will characterize temporal changes in serum and fecal acetate levels in children admitted to the intensive care unit with TBI. Clinical information including injury severity, antibiotic exposure, and diet will be collected for secondary outcomes. If we identify a robust effect and verify the need to restore acetate as a therapeutic target in humans, this will provide a foundation for future clinical trials in TBI and, in some settings such as elite athletes and the military, could even represent an opportunity for a preventative approach.